An IP multimedia subsystem (IMS) is a service delivery platform that allows service providers to deploy a single infrastructure to provide mobile and fixed multimedia services. It is a standardized next generation networking (NGN) architecture that may provide user authentication and authorization, identity management, roaming, and security and charging that can be leveraged by all session initiation protocol (SIP)-based applications. IMS is based on the concept of static rules that determine how a centralized SIP proxy will route incoming requests. For example, the request may be routed based on the registration status of a user, a matching feature tag within a SIP request or other information transported within a SIP message.
The SIP proxy model is predicated on the application of the static rules. Thus, once a static rule is set, when information that matches the static rule is determined, then the SIP message is routed based on that static rule. It is specified that messages are routed the same regardless of any changes of state that occur with the IMS. This does not provide flexibility in routing messages.